Computational Study On Acoustic Performance Of Exhaust Muffler

The exhaust noise was one of the most important noise sources of theautomobile. Muffler was used to control the exhaust noise. And it was theeffective method to reduce the noise.In today's competitive world market, it is important for a company toshorten product development cycle time in order to be successful in the targetmarkets in which they compete. Engineers can no longer rely on just the"build and test"method to design products because sometimes it simply takestoo long and is too expensive. Along with the improvement of the computerapplication level in recent years, computer aided engineering tools are oftenused by engineers to evaluate different designs quickly before buildingprototypes. With the computer simulation of exhaust mufflers and engines,the influence of mufflers on engine performance as well as exhaust noise canbe predicted. In this paper, we discuss the acoustic performance of theexhaust muffler in transmission loss, Insertion loss, and tailpipe noise.Transmission loss and insertion loss are the most frequently usedacoustic performance criteria of automotive exhaust mufflers. Transmissionloss of a muffler is usually determined and analyzed computationally andexperimentally in the development stage of an exhaust system, while insertionloss is the final acoustic performance indicator of the system.Transmission loss (TL) is one of the most frequently used criteria ofmuffler performance because it can be predicted very easily from the knownphysical parameters of a muffler and don't have effect on it for sourceimpedance and the radiation impedance, TL is a property of the muffler only.Transmission Loss Test and muffler model was built; transmission loss of anacoustic element is measured by using an extended impedance tube setup.The test piece is connected to the acoustic driver and an anechoic terminationthrough two straight pipes. It is not absolutely necessary to have extra pipelengths added to the muffler, but is used here to demonstrate good practice.Experiments often use these extra lengths for placement of the fourmicrophones. These extra lengths must have the same diameter of theconnecting pipes to prevent additional pressure loss. The test acousticelement could be a simple component or a complex exhaust system. The TLis calculated for the muffler, the muffler exhibits a broad attenuation band. The engine model must be built before the predicted of insertion loss.The model is a basic single-cylinder engine, consist of environment, intake port,intake pipe, injector, intake valve, cylinder, engine, exhaust valve, exhaust port,exhaust pipe, entering the data for these object, then connect all thecomponents together from left to right in the general flow direction. Once theengine model is fully built, the exhaust muffler is attached to the engine. Amicrophone is located at 12 inches away from the tailpipe. During the test, theengine runs from 4500 to 8000 r/min under the wide-open throttle condition. It isseen that the predicted and measured tailpipe noise match very well in lowengine speed. With the speed increased, the prediction differs from themeasured result. This is due to that at low r/min, flow speed is slow, the air flownoise dominates tailpipe noise. At high r/min, flow speed is high, the frictionnoise dominates tailpipe noise. But GT-POWER is 1-D software, it can notpredicted the friction noise. Insertion loss (IL) is defined to be the difference in the acoustic power atone point in space with and without the muffler inserted between that pointand source. For comparison and calculated insertion loss, the muffler isreplaced with one straight pipe with the same length as the corresponding...